The present invention relates to circuits for charging battery cells. More specifically, the present invention provides an apparatus for charging lithium-ion battery cells using pulse width modulation of a constant current source.
Because of their superior performance characteristics in a number of areas, lithium-ion batteries have quickly gained acceptance in portable electronics applications since their introduction in the early 1990's. Lithium-ion cells retain their charge considerably longer than comparable nickel-cadmium (NiCad) cells and are significantly smaller, both of which are desirable characteristics as consumer electronics such as, for example, cellular phones continue to decrease in size. However, in replacing NiCad technology with lithium-ion technology, there are some challenges which must be overcome with regard to all of the existing hardware configured for use with NiCad batteries. For one thing, the smaller profiles of lithium-ion batteries must be adapted to fit into devices originally designed to accommodate the larger NiCad batteries. For another, lithium-ion charger technology must be made available which rivals the cost and simplicity of NiCad charging technology, particularly where the NiCad charger comes with the consumer electronic device, i.e., a cordless phone charging base.
Unfortunately, the charging units currently available for lithium-ion cells are expensive and complex. For example, one commonly available configuration employs two control loops which precisely control both the charging voltage and current. Such chargers maintain a constant current until the battery voltage is in the desired range at which time the battery voltage is kept constant and the current is gradually folded back. By contrast, NiCad chargers are much less expensive and much simpler than their lithium-ion counterparts, often employing, for example, a poorly regulated constant current source and a voltage sensor. As a result, the consumer who upgrades a NiCad-based device to lithium-ion cells will not be able to place the consumer electronic device in its original NiCad charging stand. Rather he will have to purchase a stand-alone, dedicated lithium-ion charger at considerable additional expense.
It is desirable that consumers be able to take advantage of the superior performance of lithium-ion batteries while continuing to derive value from previously purchased consumer electronics and battery chargers. Therefore, some means to configure lithium-ion batteries for use in such applications is needed.